ColumnSet<T>

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Query;

namespace Microsoft.Xrm.Sdk.Ext
{
    public class ColumnSet<TEntity> where TEntity : Entity
    {
        private readonly ColumnSet columnSet = new ColumnSet();

        public ColumnSet(params Expression<Func<TEntity, object>>[] columns)
        {
            AddColumns(columns);
        }

        public ColumnSet(bool allColumns)
        {
            columnSet.AllColumns = allColumns;
        }

        public static implicit operator ColumnSet(ColumnSet<TEntity> cs)
        {
            return cs.columnSet;
        }

        public void AddColumns(params Expression<Func<TEntity, object>>[] columns)
        {
            foreach (var column in columns.Select(ExtractPropertyName))
            {
                AddColumn(column);
            }
        }

        public void AddColumn(Expression<Func<TEntity, object>> column)
        {
            columnSet.Columns.Add(ExtractPropertyName(column));
        }

        private static string ExtractPropertyName(Expression<Func<TEntity, object>> column)
        {
            switch (column.Body.NodeType)
            {
                case ExpressionType.MemberAccess:
                    return ((PropertyInfo)((MemberExpression)column.Body).Member).Name.ToLowerInvariant();
                case ExpressionType.Convert:
                    var expr = (MemberExpression)((UnaryExpression)column.Body).Operand;
                    return ((PropertyInfo)expr.Member).Name.ToLowerInvariant();
                default:
                    throw new NotSupportedException($"Unsupported property expression, node type: {column.Body.NodeType}");
            }
        }

        private void AddColumn(string column)
        {
            columnSet.Columns.Add(column);
        }
    }
}